Electron emitter



Oct 10, 1939. E, 2,175,696

ELECTRON EMITTER Filed May 27, 1937 INVENTOR ER/VEST ,4. LEDERER w c/m'rATTORNEY Patented Oct. 10, 1939 UNITED STATES PATENT OFFICE ELECTRONEMITTER Delaware Application May 27, 1937, Serial No. 145,050

3 Claims.

My invention relates to electron emitters, particularly thermionic orsecondary electron emitters useful in evacuated or gas filled dischargedevices and having long life and low gas content.

5 The conventional thermionic cathode comprises 8. directly orindirectly heated core of nickel coated with barium and strontiumcompoimds, such as carbonates, which can be decomposed to the oxides byheat. The carbonate coated cathl ode is mounted with other electrodes inan envelope of a discharge device, and while vacuum pumps are exhaustingthe air the metal parts of the device are heated to free them of gas anddecompose the carbonates on the cathode. Gases,

15 including large quantities of gaseous by-products of the carbonatedecomposition, are removed, a getter is flashed, the cathode isactivated by heating it at a high temperature to free some active metal.Unfortunately, high temperature opera- 20 tion after seal-off liberatesconsiderable occluded gas from the cathode core and its coating andoften evolves carbonaceous gases from undecomposed coating materialwhich, when not absorbed by the getter, return to the cathode and 25re-combine with and poison the coating. Ac-

cordingly, the amount'of active metal that can be produced in thecoating and the useful life of the cathode is seriously limited by thegas content of the cathode.

30 An object of this invention is to increase the useful life of anelectron emitter.

Another object of this invention is to reduce the gas content of anelectron emitter and increase the supply of active electron emissive 35metal.

The novel electron emitter of my invention comprises a base or core ofrefractory metal, such as nickel or any of the core metals commonly usedin cathodes, on which there is a thin ad- 40 herent layer or coating ofrefractory metal oxide,

such as magnesium oxide. The oxide coating according to my invention, isimpregnated preferably throughout the body of the oxide, and is enrichedparticularly at its surface, with an active 45 electron emitting metalsuch as barium. The coating consists of magnesium oxide particles orcrystals of such a size and shape that they are capable of retaininglarge quantities of alkaline earth metals such as metallic barium, whichdoes 50 not readily evaporate from the matrix of magnesium oxide. Theactive metal of a monotomic layer of barium on the surface of the oxideis supplied with the active metal from within the body and materiallyextends the useful life of the 55 cathode.

The invention will be more clearly understood from the followingdetailed description in connection with the accompanying drawing inwhich Figure 1 illustrates an enlarged sectional view of my improvedelectron emitter, and Figure 2 illus- 5 trates the adaptation of myimproved emitter to an indirectly heated cathode.

Referring to the drawing, a base or core l of a refractory metal, suchas nickel, is coated on its surface with a thin adherent layer 2 of ametal oxide, such as magnesium oxide. The magnesium oxide may bedeposited on the core metal in a number of ways. Finely groundcommercially pure magnesium oxide mixed with water or an organic binder,such as nitrocellulose, may be painted or sprayed upon the core andbaked. A mixture of about five grams of powdered magnesium oxide in 100c. c. of water, which incidentally produces some magnesium hydroxide,sprayed on nickel to a thickness of to 1 mil and dried at room orslightly elevated temperature for 5 minutes has been found to produce auniform adherent coating of oxide, white in color, and having low heatradiating properties.

The magnesium oxide on the core, according to my invention, isimpregnated with a good electron emissive metal 3, preferably barium.The impregnation of the magnesium oxide may conveniently be accomplishedbefore its application to the core by mixing with the oxide coatingmaterial a nitrogenous compound of barium, such as barium azide (BaNe).1'7 grams of BaNs and 5 grams of MgO mixed in 100 c. c. of water may bepainted or sprayed upon the core and dried at room or slightly elevatedtemperature. If the magnesium oxide is first applied to the core thecoating in the amorphous state may be soaked in a saturated solution ofbarium azide. The dried coating is stable in air and is particularlyresistant to contamination during handling. Another way of impregnatingthe magnesium coating with barium is byfirst spraying or coating thecore with a very thin, barely visible, coating of barium azide and thencoating with a one-half to one mil layer of magnesium oxide. 5

The core shown as the sleeve 4 of a cathode indirectly heated with aninsulated filament 5 in Figure 2 may with its magnesium oxide-bariumazide coating be mounted with cooperating electrodes in a tube envelopein the usual way. As distinguished from a tube with the conventionalbarium strontium carbonate coated cathode, in which the carbonates arebroken down at a cathode temperature of about 1250 C. and thedecomposition gases are removed by the exhaust pumps, a tube containingmy improved cathode may be degassed and sealed of! without heating thecathode above the bulb temperature. 6 milligrams of barium strontiumcarbonate on the conventional cathode used in tubes commercially knownas type 6K7 evolves during its decomposition on exhaust about 1200 litermicrons of carbon dioxide (CO1) and from. the resulting barium oxideless than 13 micrograms of barium metal may be produced. Even this smallconversion of the barium oxide to barium is obtained only by heating thecathode to a temperature of about 1250 C. for a relatively long time.The 6K7 cathode, however, coated according to my invention with a layerof magnesium oxide V2 to 1 mil thick overlying a layer of as little as40 micrograms of barium azide may be completely activated in arelatively short time at a temperature of 650 to 700 C. This improvedcathode yields about 10 liter microns of nitrogen, or only one onehundred twentieth as much gas as the conventional carbonate coatedcathode, yet produces 26 micrograms of barium, or more than 2 times asmuch barium as the carbonate cathode. The small amount of nitrogen gasliberated during activation of my improved cathode is easily absorbed bythe conventional getter and materially simplifies the exhaust scheduleof the tube.

If desired, the magnesium oxide coating may be impregnated with bariumby incorporating the barium in the core metal. A core metal of nickel,for example, alloyed with barium will when heated supply quantities ofbarium to the oxide coating. It has been found that magnesium oxide willretain metallic barium within its body and on its surface attemperatures above the vaporization temperature of barium.

Barium fluoride (BaFz) may with barium azide (BaNs) be mixed with themagnesium oxide to promote the crystallization of the oxide and increasethe conductivity of the coating.

Where the electrode constructed in accordance with my invention is to beused as a secondary electron emitter operating at low temperature, theamount of barium, particularly at the surface of the electrode, may ifdesired be greater than the amount used on a thermionic electronemitter. The film of barium upon the surface of the magnesium oxide maybe thick enough to be visible so that when the electrode is used as asecondary emitter the surface which is bombarded by primary electrons isso rich in barium that large quantities of secondary electrons areemitted.

My improved electron emitter is characterized by the small amount of gasthat is liberated during its activation and by the large amount of freemetallic barium held in reserve in the body of the oxide coating. Tubeshaving cathodes coated with barium azide impregnated magnesium oxide andexhausted as described have been found to have a useful life as great as1500 hours with only a gradual diminution in the emission after thisperiod.

I claim:

1. The method of making an electron emitter with a metal core and anelectron emitting layer comprising the steps of coating the core withmagnesium oxide and barium azide, and heating the coated core in anon-oxidizing environment to solidify and adhere the magnesium. oxide tothe core and to decompose the azide to metallic barium and disperse thebarium through the oxide.

2. The method of making an electron emitter with a metal core and anelectron emitting layer comprising the steps of coating the core withmagnesium oxide and a nitrogenous compound of barium, solidifying themagnesium oxide to adhere it to the core and heating the mixture in anoxygen free environment to decompose the

